Add exception to Admin view for Multisite installations

[dariusk]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

On a Multisite Network installation, local Admins (as opposed to "Network" AKA "Super" Admins) have reduced capabilities compared to the Admin role on standard installations. Specifically they can't install new plugins.

For Multisite users viewing the Admin page but who do not have Network Admin privileges, this commit skips over some add_filter calls that were causing errors on the plugin listing page (since the Admin did not actually have those capabilities).

This will still work correctly for a Network Admin with the plugin installation privileges since the is_network_admin() will catch that and render the correct available actions.

(co-authored by @jsdiaz)

How to test the changes in this Pull Request:

1. Install Newspack in a Multisite Network environment
2. View the plugins page as a regular Admin user (not a Network Admin)
3. Observe php errors at the top of the page
4. Apply this patch
5. View the plugins page again as the same user
6. Observe no errors

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[adekbadek]

While it works, it's a somewhat roundabout way – it'd be more readable to add a current_user_can( 'install_plugins' ) in both methods (the check won't work in the constructor. Like so: dd8946e. Would that still work for your use case @dariusk ?